When my daughter was born, she had a little strawberry nevus, or infantile haemangioma, to use the proper medical term, on her tummy. It was very small and not particularly raised: I pointed it out to the midwife, who said they were very common (especially in girls) and that it wasn't anything to worry about. I didn't think much about it after that until my niece, then seven, came to visit her new baby cousin and spied it when I was changing Amelia's nappy. 'Oh, my friend Emily's got one of those,' she said, airily. 'Last year it was tiny but now it's grown absolutely MASSIVE.'

What is a birth mark? There are several different types of birthmark, some more common than others. The two main types include, vascular birthmarks that usually appear in the head and neck area (often red, purple and pink, or occasionally blue) and are caused by abnormal blood vessels in or under the skin. Secondly, pigmented birthmarks (typically brown in colour) caused by clusters of pigment cells. Both can appear anywhere, including inside the body.

I don't normally take medical advice based on the comments of children in year 3 (advanced though my niece is, naturally) but the cool authority with which she said it made me wonder if a second opinion might be worth it after all. I wasn't the only one worrying: a couple of my NCT friends had also had babies born with birthmarks (one had a salmon patch, or 'stork bite', on her forehead; the other Mongolian spots on her upper thigh) and while the was a feeling that we didn't want to 'make a fuss' you could feel the tension thrumming through us as we debated these mysterious marks. In the end, I opted for quietly keeping an eye on it and sure enough, the mark disappeared before Amelia's third birthday.

'A strawberry nevus is a collection of blood vessels at the surface of the skin, and although one small one is nothing to worry about, they can rub against clothing or experience constant irritation which can cause them to rupture and bleed or become ulcerated,' says consultant paediatrician Dr Sunit Godambe, from the Royal College of Paediatrics and Child Health. 'If they are deeper within the body, they may not be apparent at first and appear as a lump as the haemangioma grows. They can also grow on organs inside the body, such as the liver. If your child has more than one noticeable haemangioma, you may be offered an ultrasound scan to rule this out.' More commonly, however, the course of action is to either give the child steroids (Propranolol) to shrink the birthmark, or simply wait it out. 'Most will completely disappear by the age of five,' says Dr Godambe.

Salmon patch, or stork bite

'These are tiny red marks that appear on the upper eyelid, forehead or in the centre of the nape of the neck,' says Dr Godambe. 'Around 30% of babies are born with this type of birthmark. Typically, the mark behind the neck stays while marks on the forehead and eyelids fade.'

Port wine stains

Around three in 1000 children are born with a port wine stain, which is a flat red or purple mark on the skin apparent from birth. Over time, it can thicken and become raised. 'Port wine stains develop due to an abnormal collection of blood vessels under the skin,' says Dr Godambe. 'The worry is there might be a similar structure in the brain, which can cause fits or convulsions – this is known as Sturge-Webber syndrome. If there is any suspicion that your child has this, they will be seen by a neurologist and given a brain scan, but the condition is rare.

'Because port wine stains are usually visible, a clinical psychologist is involved with the family early on to advise the parents – and later, the child – on dealing psychologically with a birthmark that singles a child out as 'different'. Laser treatment is successful at physically treating a port wine stain, and can be started when children are very small. Port wine stains are less likely to grow in adulthood (they can become bumpy and 'cobbled') if laser treatment has been carried out.'

Mongolian blue spots

'These are common in Mediterranean and Asian children and most frequently appear on the lower back or legs,' says Dr Godambe. 'They're blue-grey in colour and usually measure a few centimetres across. The can look like bruising but they're not painful to the touch, and generally get lighter and fade by the time a child reaches puberty.' Doctors think that this type of birthmark is caused by early skin cells that develop during pregnancy getting trapped deep in the layers of the skin, and they're not due to anything the mother did or didn't do.

Café au lait spots

These are coffee-coloured pigmented patches on the skin that are generally no cause for concern, although if your child develops more than six spots, or they measure more than 5cm in diameter, you should see your doctor, says Dr Sunit. 'This is because they are associated with a condition called neurofibromatosis, which causes tumours to grow on nerve tissue later on in life. Your GP will refer you to a paediatrician or dermatologist for monitoring and, if necessary, treatment.'

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